1. Field of the Invention
This invention relates in general to semiconductor devices having lightly-doped drain and source. In particular, this invention relates to semiconductor devices having drain and source with both lightly- and heavily-doped regions. More particularly, this invention relates to the process of fabricating semiconductor devices by one single implantation procedure to obtain devices having drain and source with both lightly- and heavily-doped regions.
2. Technical Background
The continuous trend towards the miniaturization of semiconductor devices has driven integrated circuits (ICs) into the sub-micron level of device fabrication. Thermal electrons trapped in the gates of the transistors in an IC device can adversely influence the reliability of the device. This effect poses a serious problem in the sub-micron category of semiconductor devices. A fabrication process for making lightly-doped drain and source regions for ICs has been proposed, such as the process exemplified here that makes the device, which is shown in cross sections in its process stages as in FIGS. 1a to 1d.
The conventional fabrication process typified in FIGS. 1a to 1d depicts, in cross sectional views, a semiconductor IC device having lightly-doped drain and source during its fabrication process stages. The fabrication can be characterized in the following stages:
First, as is shown in FIG. 1a, a semiconductor substrate 1 (of the P or N type) has a Field region OXide layer 10 (FOX) formed on its top surface which defines an active region 12. A gate oxide layer 140 and polysilicon layer 142 are further fabricated on the active region 12 to define a gate 14.
Secondly, as can be seen in FIG. 1b, impurities (N-type impurities such as P, or P-type impurities such as B) are implanted to form lightly-doped regions 160 (N- or P-implantation regions). Typically, N-type impurities are implanted into a P-type substrate while P-type impurities are implanted into a N-type substrate.
Next, FIG. 1c shows that a sidewall spacer 18 is formed on each side wall of the gate 14 by, for example, depositing a layer of oxide and then etching back by plasma etching.
Finally, as is shown in FIG. 1d, further impurities (N or P type) are implanted to form heavily-doped regions 162 (N.sup.+ or P.sup.+ implantation regions). Due to the fact that sidewalls 18 obstruct the impurities from entering into the lightly-doped regions 160 during this second implantation process, the implantation concentration in the regions 160 of drain and source 16 remain relatively light as compared to that in regions 162.
Two implantation procedures, however, in addition to the use of conventional photoresist and etching procedures, are required in the above exemplified fabrication procedure, which procedure is typically employed in the prior art. This relatively complicated fabrication procedure is not suitable for the efficient and cost-effective mass production of IC devices.